A+
Absolutely perfect. It cannot possibly be improved upon in any way.
A
Excellent—almost but not quite perfect.
A-
Still excellent, but one or more slight flaws clearly make it imperfect.
B+
Very good, but flawed enough to keep it just short of excellence.
B
Good—decidedly above average.
B-
Still good, but only slightly above average.
C+
On the high side of average, leaning toward "good."
C
Absolutely average. Not bad, but not particularly good, either.
C-
On the low side of average, leaning toward "not good."
D+
Not very good—slightly below average.
D
Definitely not good—well below average.
D-
So badly flawed that it just barely avoids being a failure.
F
Failure.